This invention relates to the production of "pelletized" aerogels by exposing a support structure to a pregel heated to hypercritical conditions. Upon venting the fluid phase under the hypercritical conditions, the aerogel forms on and/or within the support structure. The "pelletized" products produced by this process are much easier to handle than aerogels products formed by any prior art processes.
Aerogels are materials which have unusually high surface areas and pore volumes. Generally, they exhibit a very low bulk density (0.03-0.1 g/cc). Ordinarily, aerogels are obtained either as chunky (approximately 0.6-7.6 cm), but very soft (gel-like) granules, or as light dusty materials directly from an autoclave. In either form, aerogels present difficulties in handling.
Aerogels are usually produced by initially dissolving or suspending a metal ion (generally referred to as solute; usually added as the hydroxide, alkoxide or acetate) in an aqueous or alcohol (or both) medium. The medium hydrolizes and gels the metal salt to produce a pregel including a reaction liquid and the aerogel forming compound. Upon heating the pregel (aquagel or alcogel) to a temperature above the critical temperature of its aqueous or alcohol medium, the pregel is converted to the aerogel forming compound and a fluid phase. The fluid phase is then vented under hypercritical conditions to yield the aerogel. This method of reaction liquid (solvent) removal avoids the inherent shrinkage of the solid product which occurs when conventional drying techniques are employed. Aerogels have been proposed for use as catalysts, insulators, Cerenkov detectors, battery separators and solar plate collectors.
Numerous publications and patents exist which discuss metal oxide-aerogel production and use. S. J. Teichner et al., "Inorganic Oxide Aerogels," Advances in Colloid and Interface Science, Volume 5, 1976, pp 245-73, disclose the general method described supra for the preparation of inorganic oxide aerogels such as SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2, MgO and mixed oxides, and the use of certain aerogels (SiO.sub.2) as Cerenkov radiators. U.S. Pat. No. 3,963,646 (Teichner et al.) discloses the preparation of mixed oxide aerogels useful as catalysts. U.S. Pat. No. 4,268,317 (Rayl) discloses aerogels useful as insulators. U.S. Pat. No. 4,287,276 (Lundquist et al.) discloses aerogels useful as battery separators. European Pat. No. 18-955 (Von Dardel) discloses aerogels useful as solar plate collectors.
In addition to metal oxide aerogels, other compositions can be used for forming aerogels. For example, U.S. Pat. No. 3,210,273 recognizes the use of organosilica compounds for forming aerogels and U.S. Pat. No. 3,203,903 discloses the use of clays as aerogel materials.
Aerogels can be shaped into pellet form by pressing. Unfortunately, aerogels are very fragile and care must be taken to use low pressure during pelleting in order to avoid a marked decrease in pore volume. However, using low pressure to maintain a high pore volume product produces a pellet which is structurely weak and difficult to handle.
We have discovered a novel method for producing aerogels which not only aids in improving collection of the aerogels from the reaction chamber but also provides novel products which are much more suitable for handling and which can be used in a variety of applications.